The present invention relates to a capacitive load drive circuit such as a liquid crystal drive circuit, and in particular to a low power consumption, fast response capacitive load drive circuit suitable for driving a leaky capacitive load.
As a conventional circuit for driving a capacitive load, a circuit as shown in FIG. 1 is known. In FIG. 1, C denotes a capacitive load, numeral 1 a voltage output circuit for charging a capacitive load, numeral 2 a current source, numeral 3 a switch, numeral 4 control means of the switch 3, V.sub.in a signal input terminal, R a leak resistance, and V.sub.CC and V.sub.BB power sources.
A signal inputted through the signal input terminal V.sub.in is applied to the voltage output circuit 1. The output voltage of the voltage output circuit 1 changes in accordance with a change in the input signal thereof. For brevity of explanation, it is now assumed that the output voltage of the voltage output circuit illustrated in FIG. 1 is equal to the input voltage thereof. The voltage output circuit 1 of FIG. 1 has only one function of injection among two functions of charge injection and charge extraction with respect to the capacitive load C. For performing both the charge injection and the charge extraction with respect to the capacitive load C, therefore, the voltage output circuit 1 is used in combination with the current source 2 for extracting the charge. When the input signal voltage changes to a higher value under the condition that the output voltage of the voltage output circuit 1 varies in accordance with the applied input signal, the voltage across the capacitive load C is lower than the input voltage of the voltage output circuit 1. Therefore, the capacitive load C is charged until the voltage across the capacitive load C becomes equal to the input voltage of the voltage output circuit 1. On the other hand, when the input signal voltage of the voltage output circuit 1 changes to a lower value, contrary to the above described case, the voltage across the capacitive load C is higher than the input voltage of the voltage output circuit. For making the output voltage of the voltage output circuit 1 equal to the input voltage of the voltage output circuit 1, therefore, the charge of the capacitive load C must be discharged.
On the other hand, the voltage output circuit 1 is provided to charge the capacitive load C as described before, and hence the voltage output circuit is not capable of discharging the charge of the capacitive load C. When the capacitive load C is to be discharged, the switch 3 illustrated in FIG. 1 is closed to connect the capacitive load C to the current source 2, the charge of the capacitive load C being discharged. After the output voltage of the voltage output circuit 1, i.e., the voltage across the capacitive load C becomes equal to the input voltage of the voltage output circuit 1 and hence the discharge of the capacitive load is stopped, the switch 3 is opened again. The opening and closing operation of the switch 3 is controlled by the control means 4. Under the ideal condition, the voltage across the capacitive load C after the switch 3 is opened is kept at constant voltage so long as the output of the voltage output circuit 1 does not change. However, an actual capacitive load is accompanied by leak resistance as represented by R in FIG. 1, for example. In case high-precision voltage is demanded as voltage held by the capacitive load C, therefore, the voltage held by the capacitive load C is changed by the leak resistance R. This change poses a problem. For compensating this voltage change, it is necessary to always close the switch 3 to connect the capacitive load C to the current source 2 and thereby discharge the charge flowing in from the power supply V.sub.CC through the leak resistance R.
On the other hand, the current flowing into the current source is set at a large value so that the input signal voltage to the voltage output circuit 1 may be made equal to the voltage across the capacitive load C in a predetermined time even in case where the change of the output voltage of the voltage output circuit 1, i.e., the change of the voltage across the capacitive load C becomes the maximum. Since in general the leak resistance R has a large value and the voltage change of the capacitive load C caused by the leak resistance R is not so large, its compensating current may have a small value. By closing the switch 3 in the circuit of FIG. 1, therefore, a larger current than that required for compensating the voltage change of the load capacitance C caused by the leak resistance R flows, resulting in a problem of increased power dissipation. When a method of thus turning on and off one current source is used, it is difficult to make the leak current compensation compatible with the reduction in power dissipation. As a circuit of the prior art relating to this, a circuit for changing over a single current source between two values of currents is known as described in "Shuseki Kairo Kogaku (2) (Integrated Circuit Engineering (2))" written by Hisayoshi Yanai et al., published by Corona Publishing Co., Ltd., 1979, P. 97. However, this method has a problem that an element used as the current source cannot be designed in an optimum way.